DF40B(2.0)-12DS-0.4V(51) from Hirose Electric Co Ltd is a fine-pitch board-to-board mezzanine receptacle designed for compact electronic assemblies where space, stacking height, and connection reliability are critical. As part of the rectangular board-to-board connector category, it is well suited for portable devices, embedded control boards, industrial modules, medical electronics, and communication equipment that require dense interconnects in low-profile layouts. Engineers typically choose this series for precise 0.40 mm pitch routing, surface-mount assembly, and gold-finished contacts that support dependable signal transfer in tightly packed designs. The listed product status is Active, which is a positive indicator for ongoing design consideration, although lifecycle status beyond that should still be verified through the manufacturer for long-term programs. RoHS compliance information is not explicitly provided in the supplied specifications, so compliance should be treated as information not available pending official manufacturer documentation or datasheet confirmation.